In the operation of knitting machines, needles having hooks at one end are moved in reciprocating fashion to engage yarns and to pull them into loops or various structures so as to form a knitted fabric. In general, knitting machines may be classified into two types: flat bed knitting machines and circular knitting machines. In both types of machines, means is provided for urging the needles into reciprocating motion. For example, in many knitting machines the needles are provided with one or more butts that extend from the side of a medial portion of the needle shaft. This butt fits into a cam that has upper and lower faces defining a generally undulating cam track. The cam moves laterally relative to the needles, thus urging the butts to follow the cam track and thereby causing the needles to move in translation due to displacement of the cam track.
As the needles move in translation, the hooks on the ends thereof engage yarns and manipulate the yarns to form a knitted fabric. If the hook on the end of a latch needle is broken or if it fails to engage a yarn, the latch needle can no longer perform the function of forming a loop. If a broken hook is undetected, the circular knitting machine will continue to operate, but the broken hook will cause a continuing defect in the knitted fabric. These defects are unacceptable in the knitted product and therefore result in loss of salable knitted fabric. It is therefore desirable to provide a system for detecting when a needle hook is broken or otherwise nonfunctional and to stop operation of the circular knitting machine soon after the broken hook is detected so that the broken needle may be replaced.
Apparatus and methods for detecting and reducing fabric defects for use with circular knitting machines are known. For example, U.S. Pat. No. 3,785,177 (the "Hino patent") shows a device for sensing a broken yarn of a circular knitting machine. As an initial matter, the invention in the Hino patent is addressed to broken yarns, and not to broken hooks. Detecting broken yarns is particularly important in knitting machines for producing underwear and undergarments. In the Hino patent, the sensor and associated cam structure are positioned at the base of a stitch cam. Importantly, positioning of the sensor at the base of a stitch cam requires some spacing in the cam for the free deflection of needles with broken hooks or yarns. Placing the sensor at the bottom of the stitch cam, with the corresponding free space required, is impractical in modern high-speed knitting machines, which require very accurate and precise control of the needle butt during the stitch-forming process.
Other references show various apparatus and methods for detecting broken hooks in latch needles in knitting machines. Many of the references focus their detecting devices at or near the hook portion of the needle rather than at the butt portion of the needle. For example, U.S. Pat. No. 3,659,437, MacArthur et al., shows a defective needle detector that uses a beam of collimated energy directed to the hook end of the needles to detect the presence or absence of hooks. Alternatively, a magnet with a piece of electrical crystal may be mounted in the proximity of the needle hooks.
U.S. Pat. No. 3,904,529, Nakamura, shows an apparatus for detecting defective needles which uses an optical fiber having an end disposed so as to detect light reflections from hooks or latch portions of needles in a similar manner to MacArthur et al.
U.S. Pat. No. 3,905,211, Raisin, et al., shows another detector apparatus using magnetic detectors on a hosiery loom. This system employs a magnetic detector mounted to the outside of the loom, and the detector analyzes the entire needle rather than focusing on the needle hook. These and other attempts to detect broken needles and to stop operation of the machine fail to show or suggest the precepts of the present invention, particularly since they show detectors located at or near the needle hook as opposed to detectors in the cam track near the butt of the needles. Moreover, the prior art systems are more complicated and prone to malfunction. None of the systems provides for high speed operation while simultaneously maintaining precise accurate control of needles as they pass through the stitch cam.
In view of the foregoing, it is an objective of the invention to provide a simple and inexpensive improved apparatus and method for detecting broken hooks which allows for accurate control of the needle position at the stitch cam.